obj: vertex colors support in importer and exporter

Adds support for vertex colors to OBJ I/O.

Importer:

- Supports both "xyzrgb" and "MRGB" vertex color formats.
- Whenever vertex color is present in the file for a model, it is
  imported and a Color attribute is created (per-vertex, full float
  color data type). Color coming from the file is assumed to be sRGB,
  and is converted to linear upon import.

Exporter:

- Option to export the vertex colors. Defaults to "off", since not
  all 3rd party software supports vertex colors.
- When the option is "on", if a mesh has a color attribute layer,
  the active one is exported in "xyzrgb" form. If the mesh has
  per-face-corner colors, they are averaged on the vertices.
  Colors are converted from linear to sRGB upon export.

Reviewed By: Howard Trickey
Differential Revision: https://developer.blender.org/D15159

5 files changed, 46 insertions, 8 deletions

diff --git a/source/blender/io/wavefront_obj/exporter/obj_export_file_writer.cc b/source/blender/io/wavefront_obj/exporter/obj_export_file_writer.cc
index 11d1bafdafe..cb95c561547 100644
--- a/source/blender/io/wavefront_obj/exporter/obj_export_file_writer.cc
+++ b/source/blender/io/wavefront_obj/exporter/obj_export_file_writer.cc
@@ -8,7 +8,9 @@
 #include <cstdio>
 
 #include "BKE_blender_version.h"
+#include "BKE_geometry_set.hh"
 
+#include "BLI_color.hh"
 #include "BLI_enumerable_thread_specific.hh"
 #include "BLI_path_util.h"
 #include "BLI_task.hh"
@@ -241,13 +243,38 @@ void obj_parallel_chunked_output(FormatHandler<eFileType::OBJ> &fh,
 }
 
 void OBJWriter::write_vertex_coords(FormatHandler<eFileType::OBJ> &fh,
-                                    const OBJMesh &obj_mesh_data) const
+                                    const OBJMesh &obj_mesh_data,
+                                    bool write_colors) const
 {
   const int tot_count = obj_mesh_data.tot_vertices();
-  obj_parallel_chunked_output(fh, tot_count, [&](FormatHandler<eFileType::OBJ> &buf, int i) {
-    float3 vertex = obj_mesh_data.calc_vertex_coords(i, export_params_.scaling_factor);
-    buf.write<eOBJSyntaxElement::vertex_coords>(vertex[0], vertex[1], vertex[2]);
-  });
+
+  Mesh *mesh = obj_mesh_data.get_mesh();
+  CustomDataLayer *colors_layer = nullptr;
+  if (write_colors) {
+    colors_layer = BKE_id_attributes_active_color_get(&mesh->id);
+  }
+  if (write_colors && (colors_layer != nullptr)) {
+    MeshComponent component;
+    component.replace(mesh, GeometryOwnershipType::ReadOnly);
+    VArray<ColorGeometry4f> attribute = component.attribute_get_for_read<ColorGeometry4f>(
+        colors_layer->name, ATTR_DOMAIN_POINT, {0.0f, 0.0f, 0.0f, 0.0f});
+
+    BLI_assert(tot_count == attribute.size());
+    obj_parallel_chunked_output(fh, tot_count, [&](FormatHandler<eFileType::OBJ> &buf, int i) {
+      float3 vertex = obj_mesh_data.calc_vertex_coords(i, export_params_.scaling_factor);
+      ColorGeometry4f linear = attribute.get(i);
+      float srgb[3];
+      linearrgb_to_srgb_v3_v3(srgb, linear);
+      buf.write<eOBJSyntaxElement::vertex_coords_color>(
+          vertex[0], vertex[1], vertex[2], srgb[0], srgb[1], srgb[2]);
+    });
+  }
+  else {
+    obj_parallel_chunked_output(fh, tot_count, [&](FormatHandler<eFileType::OBJ> &buf, int i) {
+      float3 vertex = obj_mesh_data.calc_vertex_coords(i, export_params_.scaling_factor);
+      buf.write<eOBJSyntaxElement::vertex_coords>(vertex[0], vertex[1], vertex[2]);
+    });
+  }
 }
 
 void OBJWriter::write_uv_coords(FormatHandler<eFileType::OBJ> &fh, OBJMesh &r_obj_mesh_data) const
diff --git a/source/blender/io/wavefront_obj/exporter/obj_export_file_writer.hh b/source/blender/io/wavefront_obj/exporter/obj_export_file_writer.hh
index 77da7b44276..97c23484426 100644
--- a/source/blender/io/wavefront_obj/exporter/obj_export_file_writer.hh
+++ b/source/blender/io/wavefront_obj/exporter/obj_export_file_writer.hh
@@ -72,9 +72,11 @@ class OBJWriter : NonMovable, NonCopyable {
    */
   void write_mtllib_name(const StringRefNull mtl_filepath) const;
   /**
-   * Write vertex coordinates for all vertices as "v x y z".
+   * Write vertex coordinates for all vertices as "v x y z" or "v x y z r g b".
    */
-  void write_vertex_coords(FormatHandler<eFileType::OBJ> &fh, const OBJMesh &obj_mesh_data) const;
+  void write_vertex_coords(FormatHandler<eFileType::OBJ> &fh,
+                           const OBJMesh &obj_mesh_data,
+                           bool write_colors) const;
   /**
    * Write UV vertex coordinates for all vertices as `vt u v`.
    * \note UV indices are stored here, but written with polygons later.
diff --git a/source/blender/io/wavefront_obj/exporter/obj_export_io.hh b/source/blender/io/wavefront_obj/exporter/obj_export_io.hh
index f0263989bfc..157d7760307 100644
--- a/source/blender/io/wavefront_obj/exporter/obj_export_io.hh
+++ b/source/blender/io/wavefront_obj/exporter/obj_export_io.hh
@@ -30,6 +30,7 @@ enum class eFileType {
 
 enum class eOBJSyntaxElement {
   vertex_coords,
+  vertex_coords_color,
   uv_vertex_coords,
   normal,
   poly_element_begin,
@@ -130,6 +131,9 @@ constexpr FormattingSyntax syntax_elem_to_formatting(const eOBJSyntaxElement key
     case eOBJSyntaxElement::vertex_coords: {
       return {"v {:.6f} {:.6f} {:.6f}\n", 3, is_type_float<T...>};
     }
+    case eOBJSyntaxElement::vertex_coords_color: {
+      return {"v {:.6f} {:.6f} {:.6f} {:.6f} {:.6f} {:.6f}\n", 6, is_type_float<T...>};
+    }
     case eOBJSyntaxElement::uv_vertex_coords: {
       return {"vt {:.6f} {:.6f}\n", 2, is_type_float<T...>};
     }
diff --git a/source/blender/io/wavefront_obj/exporter/obj_export_mesh.hh b/source/blender/io/wavefront_obj/exporter/obj_export_mesh.hh
index 91213ec8152..ee2e6227700 100644
--- a/source/blender/io/wavefront_obj/exporter/obj_export_mesh.hh
+++ b/source/blender/io/wavefront_obj/exporter/obj_export_mesh.hh
@@ -241,6 +241,11 @@ class OBJMesh : NonCopyable {
     return i < 0 || i >= poly_order_.size() ? i : poly_order_[i];
   }
 
+  Mesh *get_mesh() const
+  {
+    return export_mesh_eval_;
+  }
+
  private:
   /**
    * Free the mesh if _the exporter_ created it.
diff --git a/source/blender/io/wavefront_obj/exporter/obj_exporter.cc b/source/blender/io/wavefront_obj/exporter/obj_exporter.cc
index b6e636b389d..b0938084efb 100644
--- a/source/blender/io/wavefront_obj/exporter/obj_exporter.cc
+++ b/source/blender/io/wavefront_obj/exporter/obj_exporter.cc
@@ -195,7 +195,7 @@ static void write_mesh_objects(Vector<std::unique_ptr<OBJMesh>> exportable_as_me
       auto &fh = buffers[i];
 
       obj_writer.write_object_name(fh, obj);
-      obj_writer.write_vertex_coords(fh, obj);
+      obj_writer.write_vertex_coords(fh, obj, export_params.export_colors);
 
       if (obj.tot_polygons() > 0) {
         if (export_params.export_smooth_groups) {